CRITICAL-EVALUATION OF DYES AS CRYSTAL-GROWTH INHIBITORS

Between 1933 and 1951, H. E. Buckley conducted an extensive investigat ion of the manner in which commercial dyes modify the habits of simple salt crystals. While he and others reasonably assumed that the mechan ism by which dyes inhibited the growth of crystal surfaces involved th e substitution of anionic functionalities for anion position in the la ttice, the stereochemical details of these processes were never articu lated. In light of the recent explanation, by Davey and coworkers,(1) of the inhibition of alkaline earth sulfate growth by the stereospecif ic two-point attachment of diphosphonates, we felt that the time was r ipe for a reinvestigation of Buckley's data. A conformational analysis , using the AM1 Hamiltonian, was carried out on nine constitutionally isomeric azo dyes whose habit modifying effects for five salts were re corded by Buckley. No stereochemical relationships were found which co uld explain the stated data. We argue that the action of unspecified i mpurities, often comprising 50% of commercial dye samples, may render a structural interpretation of Buckley's data virtually unattainable.